Search for anything on the system to do with fast web or my web search. If you find it remove it and reboot then try again. If not see if you get the same results with a different browser. If you still do with other browsers than you have a deep problem. If not remove original browser and rerun Ccleaner then re-install and it should be good,

Is it Windows XP? If so, and if the Kaspersky tool doesn't find anything, my only other suggestion is to boot to recovery console from a Windows XP disc and run "fixboot" and "fixmbr" commands. It will rewrite the MBR and remove any bootkit infection.

Or, if it's Windows Vista or 7, you can use "bootrec /fixmbr" and "bootrec /fixboot", either from a disc, or from the "repair my computer" startup option.

EDIT: Just noticed most of this was also listed at the bottom of the article that Jay6111 posted, but you haven't said that you already tried it.

do you think I should just back up all the clients data and reinstall the os ?

You could. Or, you could follow the instructions I posted about fixing the MBR, because even a restore wont fix that.

I don't know, Windows 7 is pretty good out of the box at preventing things from reaching the MBR and the boot sector, albeit still possible. What browser are you running? Maybe try upgrading browsers if it's possible, or downgrading and re-upgrading? The windows Malicious Software scan sometimes fixes this, or it has for me before. It's typically included in the browser upgrade software, or Windows Updates, or even standalone on the Microsoft Download website.

do you think I should just back up all the clients data and reinstall the os ?

You could. Or, you could follow the instructions I posted about fixing the MBR, because even a restore wont fix that.

I don't know, Windows 7 is pretty good out of the box at preventing things from reaching the MBR and the boot sector, albeit still possible. What browser are you running? Maybe try upgrading browsers if it's possible, or downgrading and re-upgrading? The windows Malicious Software scan sometimes fixes this, or it has for me before. It's typically included in the browser upgrade software, or Windows Updates, or even standalone on the Microsoft Download website.